1. The Field of the Invention
The present invention relates to the formation of refractory metal nitrides. More particularly, the present invention is directed to the formation of refractory metal nitrides in a process suitable for forming diffusion barriers in integrated circuit manufacturing processes.
2. The Relevant Technology
In the manufacturing of integrated circuits, barriers are often needed to prevent the diffusion of one material into another. An example of a structure which requires the use of a diffusion barrier is the CMOS memory structure of FIG. 1. Therein is shown word lines 12 formed over a silicon substrate 10. Active regions 14 underlie word lines 12, and LOCOS spacers 24 separate the illustrated memory structure from other structures formed on silicon substrate 10. Each word line 12 is connected to a corresponding memory cell 16. In order to provide access to memory cells 16, aluminum interconnect line 18 is formed on the surface of the memory structure. Separating memory cells 16 from aluminum interconnect line 18 is a passivation layer 20, which is formed of an insulating material such as borophosphosilicate glass (BPSG). To interconnect memory cells 16 with aluminum interconnect line 18, a tungsten plug 22 may be used which extends down through passivation layer 20 to make electrical contact with one of active regions 14.
The use of tungsten plug 22 is advantageous because it forms a more desirable interface with underlying active regions 14 than does aluminum. Nevertheless, the current technology used to form tungsten plug 22 does present certain drawbacks. One drawback is that the tungsten of tungsten plug 22 and the aluminum of aluminum interconnect line 18 tend to interdiffuse at temperatures of about 450.degree. C. and greater. The resulting alloy (WAl.sub.2) forms an interface between the tungsten plug and the aluminum interconnect lines that is highly resistive and hinders the performance of the memory structure or other structures connected thereto. Specifically, the alloy structure degrades the interface properties and increases the resistance of the interface, thereby lowering device speed. Device speed is an important factor in the performance of structures such as CMOS memory circuits, and consequently it is desirable to have a low resistance at the interface.
One attempt in the art to overcome this problem is the use of a titanium nitride (TiN) barrier layer which is formed over the tungsten plug before aluminum interconnect line formation. A memory structure using a titanium nitride diffusion barrier is shown in FIG. 2. Therein can be seen a titanium nitride diffusion barrier 26 located between tungsten plug structure 22 and aluminum interconnect line 18.
Titanium nitride diffusion barrier 26 performs satisfactorily as a diffusion barrier, but the formation of titanium nitride diffusion barrier 26 tends to increase the throughput time of the integrated circuit manufacturing process. In the formation of a titanium nitride diffusion barrier, titanium must first be deposited and then be annealed in an atmosphere of nitrogen. This requires several extra steps, extra equipment and space in a clean room for depositing the titanium, as well as the extra expense of the required titanium.
Experimental use has been made of tungsten nitride structures in order to overcome this problem. The use of tungsten nitride has appeared promising, as it eliminates the need for a titanium deposition step. Three methods of forming tungsten nitride diffusion barriers have been used by prior art. The first method comprises the furnace formation of tungsten nitride. The second method, similar to the first, is the rapid thermal nitridization of tungsten nitride. Both of these methods are unsuitable for applications such as CMOS memory structures, due to the high temperatures required. Diffusion barriers such as titanium nitride diffusion barrier 26 are typically formed at the back end of the memory circuit formation process at a point wherein transistors and other active devices are already formed. High temperatures tend to cause the diffusion of dopant materials from active regions 14 of transistors and other active devices, thereby lowering performance and possibly causing failure of such transistors or other active devices. Thus, the first and second methods known in the art are undesirable.
The third method known in the art for forming tungsten nitride diffusion barriers comprises plasma assisted formation of tungsten nitride, where the necessary energy for tungsten nitride formation is provided by a plasma. Plasma assisted processes are still in their infancy, and tend to be very slow and highly inefficient. Consequently, the third method is also undesirable.
Thus, it is apparent that a need exists in the art for a method for forming a diffusion barrier which overcomes the problems discussed above. It would be highly beneficial if a method could be provided for forming a diffusion barrier of tungsten nitride efficiently and at a low temperature in order to be compatible with current CMOS memory structure processes.